FIR: improve inference of implicit type arguments in casts

Type parameters do not necessarily match one-to-one, or preserve order.

compiler/testData/codegen/box/when/inferredTypeParameters.kt

@@ -0,0 +1,19 @@
+sealed class C<out T, out U>
+class A<out T>(val x: T) : C<T, Nothing>()
+class B<out U>(val x: U) : C<Nothing, U>()
+
+fun bar(x: String): C<Int, String> = B(x)
+fun baz(x: Any) = "fail: $x"
+fun baz(x: String) = x
+
+typealias Z<U> = B<U>
+
+fun box(): String =
+    when (val x = bar("O")) {
+        is A -> "fail??"
+        is B -> baz(x.x)
+    } + when (val y = bar("K")) {
+        is A -> "fail??"
+        is Z -> baz(y.x)
+        else -> "..."
+    }